The present invention relates to molds for tires, and more specifically, to an improved two-piece segmented mold.
Two-piece segmented molds include a top half and a bottom half that meet typically at an approximately axially centered parting line. Each half has a frustoconical seat which widens toward the parting line to carry a plurality of tread mold segments. The tread mold segments are installed in the seats of each of the top and bottom parts for sliding movement on guide pins or other guiding features. Top and bottom segments meet at the narrower end of the seat in the closed mold to form the tread mold ring. When the mold is opened after the cure stage, the segments slide axially inward, that is, toward the parting line, and radially outward away from the axis on the frustoconical seat. The movement of the segments is relatively away from the tire tread, which releases the molded tread from the mold segments.
In conventional two-piece segmented molds, the segments are biased by springs to assume a normally open position, that is, spaced axially inward and radially outward relative to the mold parts.
When an uncured tire is placed in the mold and the mold is closed, the segments are pushed back into contact with the mold part and with each other to form a continuous tread mold surface. A problem in the conventional design is that the sidewall of an uncured tire is soft and flexible and can be pinched in the gap between the moving tread segment and the sidewall plate when the mold is closed. This produces flashing on the tire sidewall, which is unsightly and undesirable in a cured tire. Removal of flashing requires a secondary operation which entails additional time, cost, and handling.
The invention solves this problem by providing a two-piece segmented mold in which the tread segments are biased to abut a side plate in a normally closed position, rather than being normally spaced from the side plate. In the abutting position, there is no gap between the tread segments and the side plate, which avoids pinching an uncured tire when the mold shells are closed.
According to a preferred embodiment, the segments are biased by springs acting on the tread segment, and more preferably by extension springs. According to one embodiment, the springs act between the segment and a mold back part of both mold half shells. Alternatively, the tread segments of the bottom mold half shell are biased by gravity to slide to the closed position, and the segments of the top mold half shell are biased by springs.
According to another aspect of the invention, the mold half shells include guide slots for guiding the movement of the segments between the open and closed positions. The guide slots are T-shaped, and engage a T-shaped guide bar fastened on the back of each segment. A stop plate fastened at the top of the guide slot keeps the guide bar in the slot. The guide slot and guide bar arrangement provides for smooth movement of the segments and secure positioning in the closed position.
Another aspect of the invention is a skirt mounted on the top shell or mold back and depending downward toward the bottom shell. The skirt engages the bottom shell or mold back before the mold parts completely close to form a seal around the interior of the mold. A vacuum source activates to evacuate air from the mold interior before the mold closes and before the bladder placed in the uncured tire interior is fully pressurized. Air that could have been trapped between the uncured tire and the molding surfaces is removed for better molding contact.